Blender command line open template
There is no CLI option to choose which application templates is open at startup, you need to use python to tell blender to open one of the templates.

You need to call the bpy.ops.wm.read_homefile() operator, using --python-expr in the CLI arguments should be enough - 

blender --python-expr "import bpy; bpy.ops.wm.read_homefile(app_template='usertemplate')"